It relies instead on the progressive decay or disappearance of the radioactive parent with time.

The discovery of natural Willard Libby of the United States began with his recognition that a process that had produced radiocarbon in the laboratory was also going on in Earth’s upper atmosphere—namely, the bombardment of nitrogen by free neutrons.

In addition to spatial variations of the carbon-14 level, the question of temporal variation has received much study.

A 2 to 3 percent depression of the atmospheric radioactive-carbon level since 1900 was noted soon after Libby’s pioneering work, almost certainly the result of the dumping of huge volumes of carbon-14-free carbon dioxide into the air through smokestacks.

With correction for radioactive decay during the intervening years, such old samples hopefully would show the same starting carbon-14 level as exists today. His conclusion was that over the past 5,000 years the carbon-14 level in living materials has remained constant within the 5 percent precision of measurement.

A dating method was thus available, subject only to confirmation by actual application to specific chronologic problems.

Current instrumentation is reviewed with an emphasis on background reduction features and consequent limits of detection.

In the case of LSC, the background reduction features include enhanced passive shielding, anticoincidence (active) shielding, and various forms of pulse-shape analysis.

Unlike most isotopic dating methods, the conventional carbon-14 dating technique is not based on counting daughter isotopes.Since Libby’s foundational studies, tens of thousands of carbon-14 measurements of natural materials have been made.Expressed as a fraction of the contemporary level, they have been mathematically converted to ages through Improvements in measurement accuracy and the ever-mounting experience in applying carbon-14 dating have provided superior and more voluminous data with which to better answer Libby’s original questions.It is clear that carbon-14 dates lack the accuracy that traditional historians would like to have.There may come a time when all radiocarbon ages rest on firmer knowledge of the sample’s original carbon-14 level than is now available.